They were part of it from the beginning. Hal Jordan first appeared in three issues of Showcase starting in late 1959 and then got his own series with Green Lantern in 1960. Jordan is seen getting the power ring from a dying Abin Sur in Showcase #22, the Guardians are first seen in Green Lantern #1, and Sinestro makes his first appearance in Green Lantern #7.

But when do characters outside of Abin Sur, Sinestro, and Guardians appear? From a little research, it looks like Kilowog's first appearance is in 1986. When is the first story that features more than one member of the Corp or mentions the Corp the way we know it today?

I guess what I'm getting at is that it seems like the idea of the current Corp wasn't developed until much later in the history of GL. I was wondering if that's true.

Sinestro was there from the start, but the idea that he was the one who trained Hal wasn't introduced until the "Emerald Dawn" mini-series Post-Crisis. It was such a good idea they've kept it for Hal's origin ever since except in "New Frontier" where it would have added an unnecessary sideplot to everything else.